Laser scanning apparatus and methods for thermal processing

ABSTRACT

Apparatus and methods for thermally processing a substrate with scanned laser radiation are disclosed. The apparatus includes a continuous radiation source and an optical system that forms an image on a substrate. The image is scanned relative to the substrate surface so that each point in the process region receives a pulse of radiation sufficient to thermally process the region.

CROSS REFERENCE

This application is a continuation-in-part application of theapplication having Ser. No. 10/287,864, filed Nov. 6, 2002 now U.S. Pat.No. 6,747,245.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus and methods for thermallyprocessing substrates, and in particular semiconductor substrates withintegrated devices or circuits formed thereon.

2. Description of the Prior Art

The fabrication of integrated circuits (ICs) involves subjecting asemiconductor substrate to numerous processes, such as photoresistcoating, photolithographic exposure, photoresist development, etching,polishing, and heating or “thermal processing.” In certain applications,thermal processing is performed to activate dopants in doped regions(e.g., source and drain regions) of the substrate. Thermal processingincludes various heating (and cooling) techniques, such as rapid thermalannealing (RTA) and laser thermal processing (LTP). Where a laser isused to perform thermal processing, the technique is sometimes called“laser processing” or “laser annealing.”

Various techniques and systems for laser processing of semiconductorsubstrates have been known and used in the integrated circuit (IC)fabrication industry. Laser processing is preferably done in a singlecycle that brings the temperature of the material being annealed up tothe annealing temperature and then back down to the starting (e.g.,ambient) temperature.

Substantial improvements in IC performance are possible if the thermalprocessing cycles required for activation, annealing, etc. can be keptto a millisecond or less. Thermal cycle times shorter than a microsecondare readily obtained using radiation from a pulsed laser uniformlyspread over one or more circuits. An example system for performing laserthermal processing with a pulsed laser source is described in U.S. Pat.No. 6,366,308 B1, entitled “Laser Thermal Processing Apparatus andMethod.” However the shorter the radiation pulse, the shallower theregion that can be heat-treated, and the more likely that the circuitelements themselves will cause substantial temperature variations. Forexample, a polysilicon conductor residing on a thick, field-oxideisolator is heated much more quickly than a shallow junction at thesurface of the silicon wafer.

A more uniform temperature distribution can be obtained with a longerradiation pulse since the depth of heating is greater and there is moretime available during the pulse interval for lateral heat conduction toequalize temperatures across the circuit. However, it is impractical toextend laser pulse lengths over periods longer than a microsecond andover circuit areas of 5 cm² or more because the energy per pulse becomestoo high, and the laser and associated power supply needed to providesuch high energy becomes too big and expensive.

An alternative approach to using pulsed radiation is to use continuousradiation. An example thermal processing apparatus that employs acontinuous radiation source in the form of laser diodes is disclosed inU.S. patent application Ser. No. 09/536,869, entitled “Apparatus HavingLine Source of Radiant Energy for Exposing a Substrate,” whichapplication was filed on Mar. 27, 2000 and is assigned to the sameassignee as this application. Laser diode bar arrays can be obtainedwith output powers in the 100 W/cm range and can be imaged to produceline images about a micron wide. They are also very efficient atconverting electricity into radiation. Further, because there are manydiodes in a bar each operating at a slightly different wavelength, theycan be imaged to form a uniform line image.

However, using diodes as a continuous radiation source is optimallysuited only for certain applications. For example, when annealing sourceand drain regions having a depth less than say one micron or so, it ispreferred that the radiation not be absorbed in the silicon beyond thisdepth. Unfortunately, the absorption depth for a typical laser diodeoperating at wavelength of 0.8 micron is about 20 microns for roomtemperature silicon. Thus, in thermal processing applications that seekto treat the uppermost regions of the substrate (e.g., shallower thansay one microns), most of the diode-based radiation penetrates into asilicon wafer much farther than required or desired. This increases thetotal power required. While a thin absorptive coating could be used toreduce this problem, it adds complexity to what is already a ratherinvolved manufacturing process.

SUMMARY OF THE INVENTION

An aspect of the invention is an apparatus for thermally processing aregion of a substrate. The apparatus includes a continuous radiationsource capable of providing a continuous radiation beam with a firstintensity profile and a wavelength capable of heating the substrateregion. An optical system is arranged downstream of the continuousradiation source and is adapted to receive the radiation beam and form asecond radiation beam, which forms an image at the substrate. In anexample embodiment, the image is a line image. The apparatus alsoincludes a stage adapted to support the substrate. At least one of theoptical system and the stage is adapted to scan the image with respectto the substrate in a scan direction to heat the region with a pulse ofradiation to a temperature sufficient to process the region.

Another aspect of the invention is a method of thermally processing aregion of a substrate. The method includes generating a continuous beamof radiation having a wavelength capable of heating the substrateregion, and then scanning the radiation over the region in a scandirection so that each point in the region receives an amount of thermalenergy capable of processing the substrate region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a generalized embodiment of theapparatus of the present invention;

FIG. 1B illustrates an example embodiment of an idealized line imagewith a long dimension L1 and a short dimension L2 as formed on thesubstrate by the apparatus of FIG. 1A;

FIG. 1C is a two-dimensional plot representative of the intensitydistribution associated with an actual line image;

FIG. 1D is a schematic diagram of an example embodiment of an opticalsystem for the apparatus of FIG. 1A that includes conic mirrors to forma line image at the substrate surface;

FIG. 2A is a schematic diagram illustrating an example embodiment of thelaser scanning apparatus of FIG. 1A, further including a beam converterarranged between the radiation source and the optical system;

FIG. 2B is a schematic diagram illustrating how the beam converter ofthe apparatus of FIG. 2A modifies the profile of a radiation beam;

FIG. 2C is a cross-sectional view of an example embodiment of aconverter/optical system that includes a Gaussian-to-flat-top converter;

FIG. 2D is a plot of an example intensity profile of an unvignettedradiation beam, such as formed by the converter/optical system of FIG.2C;

FIG. 2E is the plot as FIG. 2D with the edge rays vignetted by avignetting aperture to reduce the intensity peaks at the ends of image;

FIG. 3 is a schematic diagram similar to that of the apparatus of FIG.1A with additional elements representing different example embodimentsof the invention;

FIG. 4 illustrates an example embodiment of the reflected radiationmonitor of the apparatus of FIG. 3 in which the incident angle Φ isequal to or near 0°;

FIG. 5 is a close-up view of an example embodiment of the diagnosticsystem of the apparatus of FIG. 3 as used to measure the temperature ofthe substrate at or near the location of the image as it is scanned;

FIG. 6 is a profile (plot) of the relative intensity versus wavelengthfor a 1410° C. black body, which temperature is slightly above that usedto activate dopants in the source and drain regions of a semiconductortransistor;

FIG. 7 is a close-up isometric view of a substrate having featuresaligned in a grid pattern illustrating 45 degree orientation of theplane containing the incident and reflected laser beams relative to thegrid pattern features;

FIG. 8 plots the reflectivity versus incidence angle for both p and spolarization directions of a 10.6 micron laser radiation beam reflectingfrom the following surfaces: (a) bare silicon, (b) a 0.5 micron oxideisolator on top of the silicon, (c) a 0.1 micron, polysilicon runner ontop of a 0.5 micron oxide isolator on silicon, and (d) an infinitelydeep silicon oxide layer;

FIG. 9 is a top-down isometric view of an embodiment of the apparatus ofthe present invention as used to process a substrate in the form of asemiconductor wafer having a grid pattern formed thereon, illustratingoperation of the apparatus in an optimum radiation beam geometry;

FIG. 10 is a plan view of a substrate illustrating a boustrophedonicscanning pattern of the image over the substrate surface;

FIG. 11 is a cross-sectional view of an example embodiment of an opticalsystem that includes a movable scanning mirror;

FIG. 12 is a plan view of four substrates residing on a stage capable ofmoving both rotationally and linearly to perform spiral scanning of theimage over the substrates;

FIGS. 13A and 13B are plan views of a substrate illustrating analternate raster scanning pattern wherein the scan paths are separatedby a space to allow the substrate to cool before scanning an adjacentscan path;

FIG. 14 is a plot of the simulated throughput in substrates/hour vs. thedwell time in microseconds for the spiral scanning method, the opticalscanning method and the boustrophedonic scanning method for theapparatus of the present invention;

FIG. 15 is a close-up schematic diagram of an example embodiment of anLTP system similar to that of FIG. 1A, that further includes a recyclingoptical system arranged to receive reflected radiation and direct itback toward the substrate as “recycled radiation”;

FIG. 16 is a cross-sectional diagram of an example embodiment of therecycling optical system of FIG. 15 that includes a corner reflector anda collecting/focusing lens;

FIG. 17 is a cross-sectional diagram of a variation of the exampleembodiment of the recycling optical system of FIG. 16, wherein thecorner reflector is displaced (decentered) relative to the axis (AR) byan amount ΔD, resulting in an offset in the angle of incidence betweenthe directly incident and recycled radiation beams;

FIG. 18 is a cross-sectional schematic view of an example embodiment ofthe recycling optical system of FIG. 15 that includes aunit-magnification relay and a roof mirror;

FIG. 19 is a cross-sectional diagram of another example embodiment ofthe recycling optical system of FIG. 15 that includes acollecting/focusing lens and a grating; and

FIG. 20 is a cross-sectional schematic diagram of an example embodimentof an LTP system that employs two laser diode arrays and twocorresponding LTP optical systems arranged to irradiate the substrate atsimilar incidence angles from opposite sides of the substrate normal.

The various elements depicted in the drawings are merelyrepresentational and are not drawn to scale. Certain proportions thereofmay be exaggerated, while others may be minimized. The drawings areintended to illustrate various implementations of the invention, whichcan be understood and appropriately carried out by those of ordinaryskill in the art.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the embodiments of theinvention, reference is made to the accompanying drawings that form apart hereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

General Apparatus and Method

FIG. 1A is a schematic diagram of a generalized embodiment of the laserscanning apparatus of the present invention. Apparatus 10 of FIG. 1Aincludes, along an optical axis A1, a continuous radiation source 12that emits a continuous radiation beam 14A having output power and anintensity profile P1 as measured at right angles to the optical axis. Inan example embodiment, radiation beam 14A is collimated. Also in anexample embodiment, radiation source 12 is a laser and radiation beam14A is a laser beam. Further in the example embodiment, radiation source12 is a carbon dioxide (CO₂) laser operating at a wavelength betweenabout 9.4 microns and about 10.8 microns. A CO₂ laser is a veryefficient converter of electricity into radiation and its output beam istypically very coherent so that profile P1 is Gaussian. Further, theinfrared wavelengths generated by a CO₂ laser are suitable forprocessing (e.g., heating) silicon (e.g., a silicon substrate such assemiconductor wafer), as discussed below. Also in an example embodiment,radiation beam 14A is linearly polarized and can be manipulated so thatthe radiation incident on the substrate includes only a p-polarizationP, or only a s-polarization state S, or both. Because radiation source12 emits a continuous radiation beam 14A, it is referred to herein as a“continuous radiation source.” Generally, radiation beam 14A includesradiation of a wavelength that is absorbed by the substrate and istherefore capable of heating the substrate.

Apparatus 10 also includes an optical system 20 downstream fromradiation source 12 that modifies (e.g., focuses or shapes) radiationbeam 14A to form a radiation beam 14B. Optical system 20 can consist ofa single element (e.g., a lens element or a mirror) or can be made ofmultiple elements. In an example embodiment, optical system 20 may alsoinclude movable elements, such as a scanning mirror, as discussed ingreater detail below.

Apparatus 10 further includes, downstream from optical system 20, achuck 40 with an upper surface 42. Chuck 40 is supported by stage 46that in turn is supported by a platen 50. In an example embodiment,chuck 40 is incorporated into stage 46. In another example embodiment,stage 46 is movable. Further in an example embodiment, substrate stage46 is rotatable about one or more of the x, y and z axes. Chuck uppersurface 42 is capable of supporting a substrate 60 having a surface 62with a surface normal N, and an edge 63.

In an example embodiment, substrate 60 includes a reference feature 64to facilitate alignment of the substrate in apparatus 10, as describedbelow. In an example embodiment, reference feature 64 also serves toidentify the crystal orientation of a monocrystalline substrate 60. Inan example embodiment, substrate 60 is a monocrystalline silicon wafer,such as described in document #Semi M1-600, “Specifications for PolishedMonocrystalline Silicon Wafers,” available from SEMI (SemiconductorEquipment and Materials International), 3081 Zanker Road, San Jose95134, which document is incorporated by reference herein.

Further in an example embodiment, substrate 60 includes source and drainregions 66A and 66B formed at or near surface 62 as part of a circuit(e.g., transistor) 67 formed in the substrate. In an example embodiment,source and drain regions 66A and 66B are shallow, having a depth intothe substrate of one micron or less.

Axis A1 and substrate normal N form an angle Φ, which is the incidentangle φ that radiation beam 14B (and axis A1) makes with substratesurface normal N. In an example embodiment, radiation beam 14B has anincident angle φ>0 to ensure that radiation reflected from substratesurface 62 does not return to radiation source 12. Generally, theincident angle can vary over the range 0°≦φ<90°. However, certainapplications benefit from operating the apparatus at select incidentangles within this range, as described in greater detail below.

In an example embodiment, apparatus 10 further includes a controller 70coupled to radiation source 12 via a communication line (“line”) 72 andcoupled to a stage controller 76 via a line 78. Stage controller 76 isoperably coupled to stage 46 via a line 80 to control the movement ofthe stage. In an example embodiment, controller 70 is also coupled tooptical system 20 via a line 82. Controller 70 controls the operation ofradiation source 12, stage controller 76, and optical system 20 (e.g.,the movement of elements therein) via respective signals 90, 92 and 94.

In one example embodiment, one or more of lines 72, 78, 80 and 82 arewires and corresponding one or more of signals 90, 92 and 94 areelectrical signals, while in another example embodiment one or more ofthe aforementioned lines are an optical fiber and corresponding one ormore of the aforementioned signals are optical signals.

In an example embodiment, controller 70 is a computer, such as apersonal computer or workstation, available from any one of a number ofwell-known computer companies such as Dell Computer, Inc., of AustinTex. Controller 70 preferably includes any of a number of commerciallyavailable micro-processors, such as a the Intel PENTIUM series, or AMDK6 or K7 processors, a suitable bus architecture to connect theprocessor to a memory device, such as a hard disk drive, and suitableinput and output devices (e.g., a keyboard and a display, respectively).

With continuing reference to FIG. 1A, radiation beam 14B is directed byoptical system 20 onto substrate surface 62 along axis A1. In an exampleembodiment, optical system 20 focuses radiation beam 14B to form animage 100 on substrate surface 62. The term “image” is used herein togenerally denote the distribution of light formed on substrate surface62 by radiation beam 14B. Thus, image 100 does not necessarily have anassociated object in the classical sense. Further, image 100 is notnecessarily formed by bringing light rays to a point focus. For example,image 100 can be an elliptical spot formed by an anamorphic opticalsystem 20, as well as a circular spot formed a normally incident,focused beam formed from a circularly symmetric optical system. Also,the term “image” includes the light distribution formed on substratesurface 62 by intercepting beam 14B with substrate 60.

Image 100 may have any number of shapes, such as a square, rectangular,oval, etc. Also, image 100 can have a variety of different intensitydistributions, including ones that correspond to a uniform line imagedistribution. FIG. 1B illustrates an example embodiment of image 100 asa line image. An idealized line image 100 has a long dimension (length)L1, a short dimension (width) L2, and uniform (i.e., flat-top)intensity. In practice, line image 100 is not entirely uniform becauseof diffraction effects.

FIG. 1C is a two-dimensional plot representative of the intensitydistribution associated with an actual line image. For mostapplications, the integrated cross-section in the short dimension L2need only be substantially uniform in the long dimension L1, with anintegrated intensity distribution uniformity of about ±2% over theoperationally useful part of the image.

With continuing reference to FIGS. 1B and 1C, in an example embodiment,length L1 ranges from about 1.25 cm to 4.4 cm, and width L2 is about 50microns. In another example embodiment, length L1 is 1 cm or less.Further in an example embodiment, image 100 has an intensity rangingfrom 50 kW/cm² to 150 kW/cm². The intensity of image 100 is selectedbased on how much energy needs to be delivered to the substrate for theparticular application, the image width L2, and how fast the image isscanned over substrate surface 62.

FIG. 1D is a schematic diagram of an optical system 20 that includesconic mirrors M1, M2 and M3 to form a line image at the substratesurface. Optical system 20 of FIG. 1D illustrates how a segment of areflective cone can be used to focus a collimated beam into a line image100. Optical system 20 comprises, in one example embodiment, paraboliccylindrical mirror segments M1 and M2 and a conic mirror segment M3.Conic mirror segment M3 has an axis A3 associated with the whole of theconic mirror (shown in phantom). Axis A3 is parallel to collimated beam14A and lies along substrate surface 62.

Line image 100 is formed on substrate surface 62 along axis A3. Theadvantage of this arrangement for optical system 20 is that it producesa narrow, diffraction-limited image 100 with a minimal variation inincident angle φ. The length L1 of the line image depends primarily onincident angle φ and the size of the collimated beam measured in they-direction. Different incident angles φ can be achieved by switchingdifferent conic mirror segments (e.g., mirror M3′) into the path ofradiation beam 14A′. The length L1 of line image 100 can be modified bychanging the collimated beam size using, for example, adjustable (e.g.,zoom) collimating optics 104.

With continuing reference to FIG. 1D, in an example embodiment, the sizeof collimated beam 14A′ can be modified using cylindrical parabolicmirrors M1 and M2. Collimated beam 14A′ is first brought to a line focusat point F by the positive, cylindrical, parabolic mirror M1. Beforereaching the focus at point F, the focused beam 14A′ is intercepted bynegative parabolic mirror M2, which collimates the focused beam. The twocylindrical parabolic mirrors M1 and M2 change the width of thecollimated beam in y-direction only. Therefore, the parabolic mirrors M1and M2 also change the length L1 of line image 100 at substrate surface62, but not the width L2 of the line image in a direction normal to theplane of the Figure.

Also shown in FIG. 1D are alternate parabolic mirrors M1′ and M2′ and analternate conical mirror M3′, all of which can be brought intopredetermined fixed positions in the optical path using, for example,indexing wheels 106, 108 and 110.

With reference again to FIG. 1A, in an example embodiment, substratesurface 62 is scanned under image 100 using one of a number of scanningpatterns discussed in greater detail below. Scanning can be achieved ina number of ways, including by moving either substrate stage 46, orradiation beam 14B. Thus, “scanning” as the term is used herein includesmovement of the image relative to the surface of the substrate,regardless of how accomplished.

By scanning a beam of continuous radiation over substrate surface 62,e.g., over one or more select regions thereof, such as regions 66A and66B, or one or more circuits such as transistor 67, each irradiatedpoint on the substrate receives a radiation pulse. In an exampleembodiment employing a 200 microsecond dwell time (i.e., the durationthe image resides over a given point), the amount of energy received byeach scanned point on the substrate during a single scan ranges from 5J/cm² to 50 J/cm². Overlapping scans serve to further increase the totalabsorbed energy. Thus, apparatus 10 allows for a continuous radiationsource, rather than a pulsed radiation source, to be used to provide acontrolled pulse or burst of radiation to each point on a substrate withenergy sufficient to process one or more regions, e.g., circuits orcircuit elements formed therein or thereupon. Processing, as the term isused herein, includes among other things, selective melting, explosiverecrystallization, and dopant activation.

Further, as the term is used herein, “processing” does not include laserablation, laser cleaning of a substrate, or photolithographic exposureand subsequent chemical activation of photoresist. Rather, by way ofexample, image 100 is scanned over substrate 60 to provide sufficientthermal energy to raise the surface temperature of one or more regionstherein to process the one or more regions, e.g., activate dopants insource and drain regions 66A and 66B or otherwise alter the crystalstructure of the one or more regions. In an example embodiment ofthermal processing, apparatus 10 is used to quickly heat and cool, andthereby activate, shallow source and drain regions, i.e., such as sourceand drain regions 66A and 66B of transistor 67 having a depth into thesubstrate from surface 62 of one micron or less.

Apparatus 10 has a number of different embodiments, as illustrated bythe examples discussed below.

Embodiment with Beam Converter

In an example embodiment shown in FIG. 1A, profile P1 of radiation beam14A is non-uniform. This situation may arise, for example, whenradiation source 12 is a substantially coherent laser and the resultantdistribution of energy in the collimated beam is Gaussian, which resultsin a similar energy distribution when the collimated beam is imaged onthe substrate. For some applications, it may be desirable to renderradiation beams 14A and 14B into a more uniform distribution and changetheir size so that image 100 has an intensity distribution and sizesuitable for performing thermal processing of the substrate for thegiven application.

FIG. 2A is a schematic diagram illustrating an example embodiment oflaser scanning apparatus 10 of FIG. 1A that further includes a beamconverter 150 arranged along axis A1 between optical system 20 andcontinuous radiation source 12. Beam converter 150 converts radiationbeam 14A with an intensity profile P1 to a modified radiation beam 14A′with an intensity profile P2. In an example embodiment, beam converter150 and optical system 20 are combined to form a singleconverter/optical system 160. Though beam converter 150 is shown asarranged upstream of optical system 20, it could also be arrangeddownstream thereof.

FIG. 2B is a schematic diagram that illustrates how beam converter 150converts radiation beam 14A with intensity profile P1 to modifiedradiation beam 14A′ with an intensity profile P2. Radiation beams 14Aand 14A′ are shown as made up of light rays 170, with the light rayspacing corresponding to the relative intensity distribution in theradiation beams. Beam converter 150 adjusts the relative spacing (i.e.,density) of rays 170 to modify profile P1 of radiation beam 14A to formmodified radiation beam 14A′ with profile P2. In example embodiments,beam converter 150 is a dioptric, catoptric or catadioptric lens system.

FIG. 2C is a cross-sectional view of an example embodiment of aconverter/optical system 160 having a converter 150 that convertsradiation beam 14A with a Gaussian profile P1 into radiation beam 14A′with a flat-top (i.e., uniform) profile P2, and an optical system 20that forms a focused radiation beam 14B and a line image 100.Converter/focusing system 160 of FIG. 2C includes cylindrical lenses L1through L5. Here, “lenses” can mean individual lens elements or a groupof lens elements, i.e., a lens group. The first two cylindrical lensesL1 and L2 act to shrink the diameter of radiation beam 14A, whilecylindrical lenses L3 and L4 act to expand the radiation beam back toroughly its original size but with a modified radiation beam profile14A′ caused by spherical aberration in the lenses. A fifth cylindricallens L5 serves as optical system 20 and is rotated 90° relative to theother lenses so that its power is out of the plane of the figure. LensL5 forms radiation beam 14B that in turn forms line image 100 onsubstrate 60.

In an example embodiment, converter/focusing system 160 of FIG. 2C alsoincludes a vignetting aperture 180 arranged upstream of lens L1. Thisremoves the outermost rays of input beam 14A, which rays areovercorrected by the spherical aberration in the system, and which wouldotherwise result in intensity bumps on the edges of the otherwise flatintensity profile.

FIG. 2D is a plot of an example intensity profile P2 of an unvignetteduniform radiation beam 14A′ as might be formed by a typical beamconverter 150. Typically, a flat-top radiation beam profile P2 has aflat portion 200 over most of its length, and near beam ends 204includes intensity peaks 210. By removing the outer rays of the beamwith vignetting aperture 180, it is also possible to obtain a moreuniform radiation beam profile P2, as illustrated in FIG. 2E.

Although the rise in intensity at beam ends 204 can be avoided byvignetting the outermost rays of radiation beam 14A, some increase inintensity near the beam ends may be desirable to produce uniformheating. Heat is lost in the direction parallel to and normal to lineimage 100 (FIG. 1B) at beam ends 204. A greater intensity at beam ends204 thus helps to compensate for the higher heat loss. This results in amore uniform temperature profile in the substrate as image 100 isscanned over substrate 60.

Further Example Embodiments

FIG. 3 is a schematic diagram of apparatus 10 similar to that of FIG. 1Athat further includes a number of additional elements located across thetop of the figure and above substrate 60. These additional elementseither alone or in various combinations have been included to illustrateadditional example embodiments of the present invention. It will beapparent to those skilled in the art how many of the additional elementsintroduced in FIG. 3 are necessary for the operation to be performed byeach of the following example embodiments, and whether the elementsdiscussed in a previous example embodiment is also needed in theembodiment then being discussed. For simplicity, FIG. 3 has been shownto include all of the elements needed for these additional exampleembodiments since some of these embodiments do build on a previouslydiscussed embodiment. These additional example embodiments are discussedbelow.

Attenuator

With reference to FIG. 3, in one example embodiment, apparatus 10includes an attenuator 226 arranged downstream of radiation source 12 toselectively attenuate either radiation beam 14A, beam 14A′ or beam 14B,depending on the location of the attenuator. In an example embodiment,radiation beam 14A is polarized in a particular direction (e.g., p, s ora combination of both), and attenuator 226 includes a polarizer 227capable of being rotated relative to the polarization direction of theradiation beam to attenuate the beam. In another example embodiment,attenuator 226 includes at least one of a removable attenuating filter,or a programmable attenuation wheel containing multiple attenuatorelements.

In an example embodiment, attenuator 226 is coupled to controller 70 viaa line 228 and is controlled by a signal 229 from the controller.

Quarter-wave Plate

In another example embodiment, radiation beam 14A is linearly polarizedand apparatus 10 includes a quarter-wave plate 230 downstream ofradiation source 12 to convert the linear polarization to circularpolarization. Quarter-wave plate 230 works in conjunction withattenuator 226 in the example embodiment where the attenuator includespolarizer 227 to prevent radiation reflected or scattered from substratesurface 62 from returning to radiation source 12. In particular, on thereturn path, the reflected circularly polarized radiation is convertedto linear polarized radiation, which is then blocked by polarizer 227.This configuration is particularly useful where the incident angle φ isat or near zero (i.e., at or near normal incidence).

Beam Energy Monitoring System

In another example embodiment, apparatus 10 includes a beam energymonitoring system 250 arranged along axis A1 downstream of radiationsource 12 to monitor the energy in the respective beam. System 250 iscoupled to controller 70 via a line 252 and provides to the controller asignal 254 representative of the measured beam energy.

Fold Mirror

In another example embodiment, apparatus 10 includes a fold mirror 260to make the apparatus more compact or to form a particular apparatusgeometry. In an example embodiment, fold mirror 260 is movable to adjustthe direction of beam 14A′.

Further in an example embodiment, fold mirror 260 is coupled tocontroller 70 via a line 262 and is controlled by a signal 264 from thecontroller.

Reflected Radiation Monitor

With continuing reference to FIG. 3, in another example embodiment,apparatus 10 includes a reflected radiation monitor 280 arranged toreceive radiation 281 reflected from substrate surface 62. Monitor 280is coupled to controller 70 via a line 282 and provides to thecontroller a signal 284 representative of the amount of reflectedradiation 281 it measures.

FIG. 4 illustrates an example embodiment of reflected radiation monitor280 for an example embodiment of apparatus 10 in which incident angle Φ(FIGS. 1 and 2A) is equal to or near 0°. Reflected radiation monitor 280utilizes a beamsplitter 285 along axis A1 to direct a small portion ofthe reflected radiation 281 (FIG. 3) to a detector 287. Monitor 280 iscoupled to controller 70 via line 282 and provides to the controller asignal 284 representative of the detected radiation. In an exampleembodiment, a focusing lens 290 is included to focus reflected radiation281 onto detector 287.

Reflected radiation monitor 280 has several applications. In one mode ofoperation, image 100 is made as small as possible and the variation inthe reflected radiation monitor signal 284 is measured. This informationis then used to assess the variation in reflectivity across thesubstrate. This mode of operation requires that the response time of thedetector (e.g., detector 287) be equal to less than the dwell time ofthe scanned beam. The variation in reflectivity is minimized byadjusting incident angle φ by adjusting the polarization direction ofincident beam 14B, or both.

In a second mode of operation, beam energy monitoring signal 254 (FIG.3) from beam energy monitoring system 250, and the radiation monitoringsignal 284 are combined to yield an accurate measure of the amount ofabsorbed radiation. The energy in radiation beam 14B is then adjusted tomaintain the absorbed radiation at a constant level. A variation of thismode of operation involves adjusting the scanning velocity in a mannercorresponding to the absorbed radiation.

In a third mode of operation, the reflected radiation monitor signal 284is compared to a threshold, and a signal above the threshold is used asa warning that an unexpected anomaly has occurred that requires furtherinvestigation. In an example embodiment, data relating to the variationin reflected radiation is archived (e.g., stored in memory in controller70), along with the corresponding substrate identification code, toassist in determining the root cause of any anomalies found aftersubstrate processing is completed.

Diagnostic System

In many thermal processes it is advantageous to know the maximumtemperature or the temperature-time profile of the surface beingtreated. For example, in the case of junction annealing, it is desirableto very closely control the maximum temperature reached during LTP.Close control is achieved by using the measured temperature to controlthe output power of the continuous radiation source. Ideally, such acontrol system would have a response capability that is faster than, orabout as fast as, the dwell time of the scanned image.

Accordingly, with reference again to FIG. 3, in another exampleembodiment, apparatus 10 includes a diagnostic system 300 incommunication with substrate 60. Diagnostic system 300 is coupled tocontroller 70 via a line 302 and is adapted to perform certaindiagnostic operations, such as measuring the temperature of substrate62. Diagnostic system 300 provides to the controller a signal 304representative of a diagnostic measurement, such as substratetemperature.

With reference again to FIG. 4, when incident angle Φ is equal to ornear 0°, diagnostic system 300 is rotated out of the way of focusingoptical system 20.

FIG. 5 is a close-up view of an example embodiment of diagnostic system300 used to measure the temperature at or near the location of scannedimage 100. System 300 of FIG. 5 includes along an axis A2 collectionoptics 340 to collect emitted radiation 310, and a beam splitter 346 forsplitting collected radiation 310 and directing the radiation to twodetectors 350A and 350B each connected to controller 70 via respectivelines 302A and 302B. Detectors 350A and 350B detect different spectralbands of radiation 310.

A very simple configuration for diagnostic system 300 includes a singledetector, such as a silicon detector 350A, aimed so that it observes thehottest spot at the trailing edge of the radiation beam (FIG. 3). Ingeneral, signal 304 from such a detector will vary because the differentfilms (not shown) on the substrate that image 100 encounters havedifferent reflectivities. For example, silicon, silicon oxide and a thinpoly-silicon film over an oxide layer all have different reflectivitiesat normal incidence and consequently different thermal emissivities.

One way of coping with this problem is to use only the highest signalobtained over a given period of time to estimate the temperature. Thisapproach improves accuracy at the cost of reducing the response time ofthe detector.

With continuing reference to FIG. 5, in an example embodiment,collection optics 340 is focused on the trailing edge of image 100(moving in the direction indicated by arrow 354) to collect emittedradiation 310 from the hottest points on substrate 60. Thus, the hottest(i.e., highest) temperature on substrate 60 can be monitored andcontrolled directly. Control of the substrate temperature can beaccomplished in a number of ways, including by varying the power ofcontinuous radiation source 12, by adjusting attenuator 226 (FIG. 3), byvarying the substrate scanning speed or the image scanning speed, or anycombination thereof.

The temperature of substrate 60 can be gauged by monitoring emittedradiation 310 at a single wavelength, provided the entire surface 62 hasthe same emissivity. If surface 62 is patterned, then the temperaturecan be gauged by monitoring the ratio between two closely spacedwavelengths during the scanning operation, assuming the emissivity doesnot change rapidly with wavelength.

FIG. 6 is the black body temperature profile (plot) of the intensityversus wavelength for a temperature of 1410° C., which temperature wouldbe the upper limit to be used in certain thermal processing applicationsto activate dopants in the source and drain regions of a semiconductortransistor, i.e., regions 66A and 66B of transistor 67 (FIG. 3). As canbe seen from FIG. 6, a temperature approaching 1410° C. might bemonitored at 0.8 microns and 1.0 microns using detectors 350A and 350Bin the form of silicon detector arrays. An advantage of using detectorarrays as compared to single detectors is that the former allows manytemperature samples to be taken along and across image 100 so that anytemperature non-uniformities or irregularities can be quickly spotted.In an example embodiment involving the activation of dopants in sourceand drain regions 66A and 66B, the temperature needs to be raised to1400° C. with a point-to-point maximum temperature variation of lessthan 10° C.

For temperature control in the 1400° C. region, the two spectral regionsmight be from 500 nm to 800 nm and from 800 nm to 1100 nm. The ratio ofthe signals from the two detectors can be accurately related totemperature, assuming that the emissivity ratios for the two spectralregions does not vary appreciably for the various materials on thesubstrate surface. Using a ratio of the signals 304A and 304B fromsilicon detectors 350A and 350B for temperature control makes itrelatively easy to achieve a control-loop bandwidth having a responsetime roughly equal to the dwell time.

An alternate approach is to employ detectors 350A and 350B in the formof detector arrays, where both arrays image the same region of thesubstrate but employ different spectral regions. This arrangementpermits a temperature profile of the treated area to be obtained andboth the maximum temperature and the temperature uniformity to beaccurately assessed. This arrangement also permits uniformityadjustments to the intensity profile. Employing silicon detectors insuch an arrangement allows for a control-loop bandwidth having aresponse time roughly equal to the dwell time.

Another method to compensate for the varying emissivity of the filmsencountered on the substrate is to arrange diagnostic system 300 suchthat it views substrate surface 62 at an angle close to Brewster's anglefor silicon using p-polarized radiation. In this case, Brewster's angleis calculated for a wavelength corresponding to the wavelength sensed bydiagnostic system 300. Since the absorption coefficient is very nearlyunity at Brewster's angle, so also is the emissivity. In an exampleembodiment, this method is combined with the methods of taking signalratios at two adjacent wavelengths using two detector arrays. In thiscase, the plane containing the viewing axis of diagnostic system 300would be at right angles to the plane 440 containing the radiation beam14B and the reflected radiation 281, as illustrated in FIG. 7.

Scanned image 100 can produce uniform heating over a large portion ofthe substrate. However, diffraction, as well as a number of possibledefects in the optical train, can interfere with the formation of theimage and cause an unanticipated result, such as non-uniform heating.Thus, it is highly desirable to have a built-in image monitoring systemthat can directly measure the energy uniformity in the image.

An example embodiment of an image monitoring system 360 is illustratedin FIG. 5. In an example embodiment, image monitoring system 360 isarranged in the scanning path and in the plane PS defined by substratesurface 62. Image monitoring system 360 includes a pinhole 362 orientedin the scan path, and a detector 364 behind the pinhole. In operation,substrate stage 46 is positioned so that detector 364 samples image 100representative of what a point on the substrate might see during atypical scan of the image. Image monitoring system 360 is connected tocontroller 70 via a line 366 and provides a signal 368 to the controllerrepresentative of the detected radiation.

Sampling portions of the image provides the data necessary for the imageintensity profile (e.g., FIG. 1C) to be determined, which in turn allowsfor the heating uniformity of the substrate to be determined.

Substrate Pre-aligner

With reference again to FIG. 3, in certain instances, substrate 60 needsto be placed on chuck 40 in a predetermined orientation. For example,substrate 60 can be crystalline (e.g., a crystalline silicon wafer). Theinventors have found that in thermal processing applications utilizingcrystalline substrates it is often preferred that the crystal axes bealigned in a select direction relative to image 100 to optimizeprocessing.

Accordingly, in an example embodiment, apparatus 10 includes apre-aligner 376 coupled to controller 70 via a line 378. Pre-aligner 376receives a substrate 60 and aligns it to a reference position P_(R) bylocating reference feature 64, such as a flat or a notch, and moving(e.g., rotating) the substrate until the reference feature aligns withthe select direction to optimize processing. A signal 380 is sent tocontroller 70 when the substrate is aligned. The substrate is thendelivered from the pre-aligner to chuck 40 via a substrate handler 386,which is in operative communication with the chuck and pre-aligner 376.Substrate handler 386 is coupled to controller 70 via a line 388 and iscontrolled via a signal 390. Substrate 60 is then placed on chuck 40 ina select orientation corresponding to the orientation of the substrateas pre-aligned on pre-aligner 376.

Measuring the Absorbed Radiation

By measuring the energy in one of radiation beams 14A, 14A′ or 14B usingbeam energy monitoring system 250, and from measuring the energy inreflected radiation 281 using monitoring system 280, the radiationabsorbed by substrate 60 can be determined. This in turn allows theradiation absorbed by substrate 60 to be maintained constant duringscanning despite changes in the reflectance of substrate surface 62. Inan example embodiment, maintaining a constant energy absorption per unitarea is accomplished by adjusting one or more of the following: theoutput power of continuous radiation source 12; the scanning speed ofimage 100 over substrate surface 62; and the degree of attenuation ofattenuator 226.

In an example embodiment, constant energy absorption per unit area isachieved by varying the polarization of radiation beam 14B, such as byrotating quarter wave plate 230. In another example embodiment, theenergy absorbed per unit area is varied or maintained constant by anycombination of the above mentioned techniques. The absorption in siliconof select infrared wavelengths is substantially increased by dopantimpurities that improve the electrical conductivity of the silicon. Evenif minimal absorption of the incident radiation is achieved at roomtemperature, any increase in temperature increases the absorption,thereby producing a runaway cycle that quickly results in all theincident energy being absorbed in a surface layer only a few micronsdeep.

Thus, the heating depth in a silicon wafer is determined primarily bydiffusion of heat from the surface of the silicon rather than by theroom-temperature absorption depth of the infrared wavelengths. Also,doping of the silicon with n-type or p-type impurities increases theroom temperature absorption and further promotes the runaway cycleleading to strong absorption in the first few microns of material.

Incident Angle at or Near Brewster's Angle

In an example embodiment, incident angle φ is set to correspond toBrewster's angle. At Brewster's angle all the p-polarized radiation P(FIG. 3) is absorbed in substrate 60. Brewster's angle depends on therefractive index of the material on which the radiation is incident. Forexample, Brewster's angle is 73.69° for room temperature silicon and awavelength λ=10.6 microns. Since about 30% of the incident radiationbeam 14B is reflected at normal incidence (φ=0), using p-polarizationradiation at or near Brewster's angle can significantly reduce the powerper unit area required to perform thermal processing. Using a relativelylarge incident angle φ such as Brewster's angle also broadens the widthof image 100 in one direction by cos⁻¹ φ, or by about 3.5 times that ofthe normal incidence image width. The effective depth of focus of image100 is also reduced by a like factor.

Where substrate 60 has a surface 62 with a variety of different regionssome of which have multiple layers, as is typically the case forsemiconductor processing for forming ICs, the optimum angle forprocessing can be gauged by plotting the reflectivity versus incidentangle Φ for the various regions. Generally it will be found that forp-polarized radiation that a minimum reflectivity occurs for everyregion near Brewster's angle for the substrate. Usually an angle, or asmall range of angles, can be found that both minimizes and equalizesthe reflectivity of each region.

In an example embodiment, incident angle Φ is confined within a range ofangles surrounding Brewster's angle. For the example above whereBrewster's angle is 73.69°, the incident angle φ may be constrainedbetween 65° and 80°.

Optimizing the Radiation Beam Geometry

Thermally processing substrate 60 by scanning image 100 over surface 62,in an example embodiment, causes a very small volume of material at thesubstrate surface to be heated close to the melting point of thesubstrate. Accordingly, a substantial amount of stress and strain iscreated in the heated portion of the substrate. Under is someconditions, this stress results in the creation of undesirable slipplanes that propagate to surface 62.

Also, in an example embodiment radiation beam 14A is polarized. In sucha case, it is practical to choose the direction of polarization of theincident radiation beam 14B relative to substrate surface 62, as well asthe direction of radiation beam 14B incident on surface 62 that resultsin the most efficient processing. Further, thermal processing ofsubstrate 60 is often performed after the substrate has been through anumber of other processes that alter the substrate properties, includingthe structure and topography.

FIG. 7 is a close-up isometric view of an example substrate 60 in theform of a semiconductor wafer having a pattern 400 formed thereon. In anexample embodiment, pattern 400 contains lines or edges 404 and 406conforming to a grid (i.e., a Manhattan geometry) with the lines/edgesrunning in the X- and Y-directions. Lines/edges 404 and 406 correspond,for example, to the edges of poly-runners, gates and field oxideisolation regions, or IC chip boundaries. Generally speaking, in IC chipmanufacturing the substrate is patterned mostly with features running atright angles to one another.

Thus, for example, by the time substrate (wafer) 60 has reached thepoint in the process of forming an IC chip where annealing or activationof the source and drain regions 66A and 66B is required, surface 62 isquite complex. For example, in a typical IC manufacturing process, aregion of surface 62 may be bare silicon, while another region of thesurface may have a relatively thick silicon oxide isolation trench,while yet other regions of the surface may have a thin polysiliconconductor traversing the thick oxide trench.

Accordingly, if care is not taken, line image 100 can be reflected ordiffracted from some sections of substrate surface 62, and can beselectively absorbed in others, depending on the surface structure,including the dominant direction of the lines/edges 404 and 406. This isparticularly true in the embodiment where radiation beam 14B ispolarized. The result is non-uniform substrate heating, which isgenerally undesirable in thermal processing.

Thus, with continuing reference to FIG. 7, in an example embodiment ofthe invention, it is desirable to find an optimum radiation beamgeometry, i.e., a polarization direction, an incident angle φ, a scandirection, a scan speed, and an image angle θ, that minimizes variationsin the absorption of radiation beam 14B in substrate 60. It is furtherdesirable to find the radiation beam geometry that minimizes theformation of slip planes in the substrate.

Point-to-point variations in radiation 281 reflected from, substrate 60are caused by a number of factors, including film compositionvariations, the number and proportion of lines/edges 404 and 406, theorientation of the polarization direction, and the incident angle φ.

With continuing reference to FIG. 7, a plane 440 is defined as thatcontaining radiation beam 14B and reflected radiation 281. The variationin reflection due to the presence of lines/edges 404 and 406 can beminimized by irradiating the substrate with radiation beam 14B such thatplane 440 intersects substrate surface 62 at 45° to lines/edges 404 and406. The line image is formed so that its long direction is also eitheraligned in the same plane 440 or is at right angles to this plane. Thus,regardless of the incident angle φ, the image angle θ between line image100 and respective lines/edges 404 and 406 is 45°.

The variations in the amount of reflected radiation 281 due to thevarious structures on substrate surface 62 (e.g., lines/edges 404 and406) can be further reduced by judiciously selecting incident angle φ.For example, in the case of forming a transistor as part of an IC, whena substrate 60 is ready for annealing or activation of source and drainregions 66A and 66B, it will typically contain all of the followingtopographies: a) bare silicon, b) oxide isolators (e.g., about 0.5microns thick) buried in the silicon, and c) a thin (e.g., 0.1 micron)polysilicon runners on top of buried oxide isolators.

FIG. 8 is sets of plots of the room-temperature reflectivity for bothp-polarization P and s-polarization S for a 10.6 micron wavelength laserradiation for each of the above-mentioned topographies atop an undopedsilicon substrate, along with the reflectivity for a infinitely deepsilicon dioxide layer. It is readily apparent from FIG. 8 that thereflectivity varies greatly depending on the polarization and theincident angle φ.

For the p-polarization P (i.e., polarization in plane 440) with incidentangles φ between about 65° and about 80°, the reflectivity for all fourcases is a minimum, and the variation from case to case is also aminimum. Thus, the range of incident angles φ from about 65° to about80° is particularly well suited for apparatus 10 for thermallyprocessing a semiconductor substrate (e.g., activating doped regionsformed in a silicon substrate), since it minimizes both the total powerrequired and the point-to-point variation in absorbed radiation.

The presence of dopants or higher temperatures renders the silicon morelike a metal and serves to shift the minimum corresponding to Brewster'sangle to higher angles and higher reflectivities. Thus, for dopedsubstrates and/or for higher temperatures, the optimum angle will behigher than those corresponding the Brewster's angle at room temperaturefor undoped material.

FIG. 9 is a top-down isometric view of apparatus 10 used to process asubstrate 60 in the form of a semiconductor wafer, illustratingoperation of the apparatus in an optimum radiation beam geometry. Wafer60 includes grid pattern 400 formed thereon, with each square 468 in thegrid representing, for example, an IC chip (e.g., such as circuit 67 ofFIG. 1A). Line image 100 is scanned relative to substrate (wafer)surface 62 in a direction 470 that results in an image angle θ of 45°.

Accounting for Crystal Orientation

As mentioned above, crystalline substrates, such as monocrystallinesilicon wafers, have a crystal planes whose orientation is oftenindicated by reference feature 64 (e.g., a notch as shown in FIG. 9, ora flat) formed in the substrate at edge 63 corresponding to thedirection of one of the major crystal planes. The scanning of line image100 generates large thermal gradients and stress concentrations in adirection 474 normal to scan direction 470 (FIG. 9), which can have anadverse effect on the structural integrity of a crystalline substrate.

With continuing reference to FIG. 9, the usual case is for a siliconsubstrate 60 having a (100) crystal orientation and lines/edges 404 and406 aligned at 45° to the two principle crystal axes (100) and (010) onthe surface on the wafer. A preferred scan direction is along one of theprinciple crystal axes to minimize the formation of slip planes in thecrystal. Thus the preferred scan direction for minimizing slipgeneration in the crystal also coincides with the preferred directionwith respect to lines/edges 404 and 406 in the usual case for a siliconsubstrate. If a constant orientation is to be maintained between lineimage 100, lines/edges 404 and 406, and the crystal axes (100) and(010), then the scanning of the line image with respect to the substrate(wafer) 60 must be performed in a linear (e.g., back and forth) fashionrather than in a circular or arcuate fashion. Also, since a specificscan direction is desired with respect to the crystal orientation, in anexample embodiment the substrate is pre-aligned on chuck 40 using, forexample, substrate pre-aligner 376 (FIG. 3).

By carefully choosing the orientation between the substrate crystal axes(100) and (010) and scan direction 470, it is possible to minimize thelikelihood of producing slip planes in the substrate crystalline latticedue to thermally induced stress. The optimal scan direction wherein thecrystal lattice has maximum resistance to slip induced by a steepthermal gradient is believed to vary depending on the nature of thecrystal substrate. However, the optimal scan direction can be foundexperimentally by scanning image 100 in a spiral pattern over a singlecrystal substrate and inspecting the wafer to determine which directionswithstand the highest temperature gradients before exhibiting slip.

In a substrate 60 in the form of a (100) crystal silicon wafer, theoptimal scan direction is aligned to the (100) substrate crystal latticedirections or at 45° to the pattern grid directions indicated bylines/edges 404 and 406. This has been experimentally verified by theinventors by scanning a radially-oriented line image 100 in a spiralpattern that gradually increases the maximum temperature as a functionof distance from the center of the substrate. The optimal scan directionwas determined by comparing the directions exhibiting the greatestimmunity to slipping with the directions of the crystal axes.

Image Scanning

Boustrophedonic Scanning

FIG. 10 is a plan view of a substrate illustrating a boustrophedonic(i.e., alternating back and forth or “X-Y”) scanning pattern 520 ofimage 100 over substrate surface 62 to generate a short thermal pulse ateach point on the substrate traversed by the image. Scanning pattern 520includes linear scanning segments 522. Boustrophedonic scanning pattern520 can be carried out with a conventional bidirectional, X-Y stage 46.However, such stages typically have considerable mass and limitedacceleration capability. If a very short dwell time (i.e., the durationthe scanned image resides over a given point on the substrate) isdesired, then a conventional stage will consume a considerable amount oftime accelerating and decelerating. Such a stage also takes upconsiderable space. For example, a 10 microsecond dwell time with a 100micron beam width would require a stage velocity of 10 meters/second(m/s). At an acceleration of 1 g or 9.8 m/s², it would take 1.02 secondsand 5.1 meters of travel to accelerate/decelerate. Providing 10.2 metersof space for the stage to accelerate and decelerate is undesirable.

Optical Scanning

The scanning of image 100 over substrate surface 62 may be performedusing a stationary substrate and a moving image, by moving the substrateand keeping the image stationary, or a moving both the substrate theimage.

FIG. 11 is a cross-sectional view of an example embodiment of an opticalsystem 20 that includes a movable scanning mirror 260. Very higheffective acceleration/deceleration rates (i.e., rates at which a stagewould need to move to achieve the same scanning effect) can be achievedusing optical scanning.

In optical system 20 of FIG. 11, radiation beam 14A (or 14A′) isreflected from scanning mirror 260 located at the pupil of an f-thetarelay optical system 20 made from cylindrical elements L10 through L13.In an example embodiment, scanning mirror 260 is coupled to and drivenby a servo-motor unit 540, which is coupled to controller 70 via line542. Servo unit 540 is controlled by a signal 544 from controller 70 andcarried on line 542.

Optical system 20 scans radiation beam 14B over substrate surface 62 toform a moving line image 100. Stage 46 increments the substrate positionin the cross-scan direction after each scan to cover a desired region ofthe substrate.

In an example embodiment, lens elements L10 through L13 are made of ZnSeand are transparent to both the infrared wavelengths of radiationemitted by a CO₂ laser, and the near-IR and visible radiation emitted bythe heated portion of the substrate. This permits a dichroicbeam-splitter 550 to be placed in the path of radiation beam 14Aupstream of scan mirror 260 to separate the visible and near IRwavelengths of radiation emitted from the substrate from the longwavelength radiation of radiation beam 14A used to heat the substrate.

Emitted radiation 310 is used to monitor and control the thermalprocessing of the substrate and is detected by a beam diagnostic system560 having a collection lens 562 and a detector 564 coupled tocontroller 70 via line 568. In an example embodiment, emitted radiation310 is filtered and focused onto separate detector arrays 564 (only oneis shown). A signal 570 corresponding to the amount of radiationdetected by detector 564 is provided to controller 70 via line 568.

Although FIG. 11 shows radiation beam 14B having an incident angle φ=0,in other embodiments the incident angle is φ>0. In an exampleembodiment, incident angle φ is changed by appropriately rotatingsubstrate stage 46 about an axis AR.

An advantage of optical scanning is it can be performed at very highspeeds so that a minimum amount of time is lost accelerating anddecelerating the beam or the stage. With commercially available scanningoptical systems, it is possible to achieve the equivalent of an 8000 gstage-acceleration.

Spiral Scanning

In another example embodiment, image 100 is scanned relative tosubstrate 60 in a spiral pattern. FIG. 12 is a plan view of foursubstrates 60 residing on stage 46, wherein the stage has the capabilityof moving both rotationally and linearly with respect to image 100 tocreate a spiral scanning pattern 604. The rotational motion is about acenter of rotation 610. Also, stage 46 is capable of carrying multiplesubstrates, with four substrates being shown for the sake ofillustration.

In an example embodiment, stage 46 includes a linear stage 612 and arotational stage 614. Spiral scanning pattern 604 is formed via acombination of linear and rotational motion of the substrates so thateach substrate is covered by part of the spiral scanning pattern. Tokeep the dwell time constant at each point on the substrates, therotation rate is made inversely proportional to the distance of image100 from center of rotation 610. Spiral scanning has the advantage thatthere is no rapid acceleration/deceleration except at the beginning andend of the processing. Accordingly, it is practical to obtain shortdwell times with such an arrangement. Another advantage is that multiplesubstrates can be processed in a single scanning operation.

Alternate Raster Scanning

Scanning image 100 over substrate 60 in a boustrophedonic pattern with asmall separation between adjacent path segments can result inoverheating the substrate at the end of a scan segment where one segmenthas just been completed and a new one is starting right next to it. Insuch a case, the beginning portion of the new scan path segment containsa significant thermal gradient resulting from the just-completed scanpath segment. This gradient raises the temperature produced by the newscan unless the beam intensity is appropriately modified. This makes itdifficult to achieve a uniform maximum temperature across the entiresubstrate during scanning.

FIGS. 13A and 13B are plan views of a substrate 60 illustrating analternate raster scanning path 700 having linear scanning path segments702 and 704. With reference first to FIG. 13A, in the alternate rasterscanning path 700, scanning path segments 702 are first carried out sothat there is a gap 706 between adjacent scanning paths. In an exampleembodiment, gap 706 has a dimension equal to some integer multiple ofthe effective length of the line scan. In an example embodiment, thewidth of gap 706 is the same as or close to length L1 of image 100.Then, with reference to FIG. 13B, scanning path segments 704 are thencarried out to fill in the gaps. This scanning method drasticallyreduces the thermal gradients in the scan path that arise withclosely-spaced, consecutive scan path segments, making it easier toachieve a uniform maximum temperature across the substrate duringscanning.

Throughput Comparison of Scanning Patterns

FIG. 14 is a plot of the simulated throughput (substrates/hour) vs. thedwell time (seconds) for the spiral scanning method (curve 720), theoptical scanning method (curve 724) and the boustrophedonic (X-Y)scanning method (curve 726). The comparison assumes an exampleembodiment with a 5 kW laser as a continuous radiation source used toproduce a Gaussian beam and thus a Gaussian image 100 with a beam widthL2 of 100 microns scanned in overlapping scan paths to achieve aradiation uniformity of about ±2%.

From the plot, it is seen that the spiral scanning method has betterthroughput under all conditions. However, the spiral scanning methodprocesses multiple substrates at one time and so requires a largesurface capable of supporting 4 chucks. For example, for four 300 mmwafers, the surface would be larger than about 800 mm in diameter.Another disadvantage of this method is that it cannot maintain aconstant direction between the line scan image and the crystalorientation of the substrate, so that it cannot maintain an optimumprocessing geometry for a crystalline substrate.

The optical scanning method has a throughput that is almost independentof dwell time and has an advantage over the X-Y stage scanning systemfor short dwell times requiring high scanning speeds.

Recycling Optical System

In the present invention, it is important to transfer as much energy aspossible from continuous radiation source 12 to substrate 60.Accordingly, with reference briefly to FIG. 19, discussed in greaterdetail below, in an example embodiment, radiation beam 14B has asubstantial range of incident angles at the substrate. That is to say,optical system 20 has a substantial numerical aperture NA=sin θ_(14B),wherein θ_(14B) is the half-angle formed by axis A1 and the outer rays15A or 15B of radiation beam 14B. Note that incident angle φ_(14B) ismeasured between the surface normal N and axis A1, wherein axis A1 alsorepresents an axial ray of radiation beam 14B. The angle φ_(14B) formedby axial ray (axis A1) and the substrate surface normal N is referred toherein as the “central angle” of the range of angles provided byradiation beam 14B.

In an example embodiment, the central angle φ_(14B) is selected tominimize the variation of reflectivity between the various film stacks(not shown) on the substrate.

In practice, it is difficult to prevent a portion of radiation beam 14Bfrom reflecting from substrate surface 62. Thus, an example embodimentof the present invention involves capturing reflected radiation 23R andredirecting it back toward the substrate as “recycled radiation” 23RD,where it can be absorbed by the substrate at the location where incidentbeam 14B was reflected. The recycled radiation 23RD further contributesto the annealing process by providing additional heat to one or moresubstrate regions (e.g., regions 66A, 66B of FIG. 1A) to be processed.

Accordingly, with reference now to FIG. 15, there is shown a close-upschematic diagram of an example embodiment of the laser scanningapparatus 10 of the present invention. Apparatus 10 of FIG. 15 issimilar to that of FIG. 1A, however it further includes a recyclingoptical system 900 arranged to receive reflected radiation 23R andredirect it back to the substrate as recycled radiation 23RD. Recyclingoptical system 900 is arranged along an axis AR that makes an angleφ_(23RD) relative to surface normal N. In order for recycling system 900to best receive reflected radiation 23R, in an example embodiment angleφ_(23RD) is made equal to radiation beam incident angle φ_(14B).

It should be noted that in the present invention, the substrate isirradiated with a pulse of radiation. As described above, the radiation“pulse” is created by scanning the substrate relative to radiation beam14B so that select portions of the substrate are exposed to radiationbeam 14B for a given amount of time, i.e., the dwell time of the beam.Strictly speaking, in the embodiment of apparatus 10 with recyclingoptical system 900, reflected radiation 23RD actually constitutes asecond pulse of light weaker than the pulse associated with incidentradiation 14B. This second pulse is time delayed from the first pulse anamount of time ΔT=OPL/c, where OPL is the optical path length reflectedlight 23R travels in optical system 900 before returning to thesubstrate, and c is the speed of light (˜3×10⁸ m/s).

Since the OPL is on the order of a meter or less, the time delay ΔTbetween pulses is on the order of 10⁻⁹ second. As the scan speeds are onthe order of 1 meter/second (m/s), the spatial separation of the firstand second pulses at the substrate surface 62 is given by ˜(1 m/s)(10⁻⁹s)˜10⁻⁹ m, which is an insignificant spatial separation in the contextof laser annealing. Thus, the incident and reflected radiation pulseseffectively overlap, i.e., they reach the same portion of the substrateat the same time for all practical purposes. Thus, the combination ofthe incident and reflected pulses results in a single energy-enhancedradiation pulse. Stated differently, for all intents and purposes, theincident (first) radiation beam 14B and the recycled (second) radiationbeam 23RD irradiate the substrate (e.g., one or more regions thereon)simultaneously.

FIG. 16 is a cross-sectional diagram of an example embodiment ofrecycling optical system 900 that includes a hollow corner cubereflector 910 and a collecting/focusing lens 916 having a focal length Fthat corresponds to the distance from the lens to substrate surface 62along axis AR. Hollow corner cube reflector 910 has three reflectingsurfaces that intersect at right angles, although to simplify thedrawing only 2 surfaces, 912 and 914, are shown schematically in FIG.16.

In the operation of optical system 900 of FIG. 16, lens 916 collectsreflected radiation 23R from substrate surface 62 and directs it tocorner cube reflector surfaces 912 and 914 as parallel rays 920. Theparallel rays reflect from the three reflector surfaces and are directedback to lens 916 in exactly the opposite direction, on the opposite sideof axis AR, as parallel rays 920′ that now constitute recycled radiation23RD. Parallel rays 920′ are collected by lens 916 and are refocused atsubstrate surface 62 back at their point of origin 321.

FIG. 17 is a cross-sectional diagram of a variation of the exampleembodiment illustrated in FIG. 16, wherein corner cube reflector 910 isdisplaced (decentered) relative to axis AR by an amount ΔD. This resultsin an offset in the angle of incidence at the substrate betweenreflected radiation beam 23R and recycled radiation beam 23RD. Note thatthe position of the beam on the substrate remains the same—only theincidence angle changes. A relative offset between the incidence anglesof the two beams can be exploited to prevent reflected radiation fromtraveling back up into continuous radiation source 12 (FIG. 15). In thisparticular example embodiment, a refractive corner cube that employstotal internal reflection is not preferred because it does not preservethe polarization of the beam.

FIG. 18 is a cross-sectional diagram of another example embodiment ofrecycling optical system 900. It includes, in order along axis AR fromsubstrate 60, a cylindrical mirror 950, a first cylindrical lens 352, apupil 954, a second cylindrical lens 956, and a tiltedpolarization-preserving roof mirror 960. In an example embodiment, firstand second cylindrical lenses 352 and 956 have the same focal length(F′) and are separated by twice their focal length (2F′) and constitutea 1× relay with pupil 954 half-way in between. Roof mirror 960 islocated F′ away from cylindrical lens 956 and the roof-line is orientedwith the direction of the p-polarized radiation it is reflecting.

In the example embodiment of recycling optical system 900 of FIG. 18, itis assumed that radiation beam 14B is focused by optical system 20 toform line image 100 on the substrate (FIG. 15). Cylindrical mirror 950receives and collimates reflected radiation 23R, which then passesthrough cylindrical lenses 352 and 956. Roof mirror 960 is arranged toredirect the radiation back through the cylindrical lenses, to thecylindrical mirror, and back to the substrate surface. The tilt on roofmirror 960 with respect to incident radiation beam 23 determines theangle of incidence of the redirected preheating radiation beam 23RD ontosubstrate 60. In an example embodiment, polarization-preserving roofmirror 960 includes a small tilt designed to keep the recycled radiation23RD from returning to continuous radiation source 12. Radiationreturned to the resonating cavity of a laser or laser diode can causeoperational problems, such as instabilities in the output power level ofthe laser.

FIG. 19 is a cross-sectional diagram of another example embodiment ofrecycling optical system 900 that includes a collecting/focusing lens1050 and a grating 1060 having a grating surface 1062. In an exampleembodiment, lens 1050 is a high-resolution, telecentric relay havingfirst and second lenses 1070 and 1072 and an aperture stop 1074 locatedbetween the first and second lenses. Further in the example embodiment,lens 1050 has a focal length F1 at the substrate side and a focal lengthF2 at the grating side, and the lenses are arranged such that substratesurface 62 is located a distance F1 away from lens 1070 as measuredalong axis AR, and grating 1060 is located a distance F2 away from lens1072 as measured along the axis AR. Also the two lenses, 1070 and 1072,are separated by a distance equal to the sum of their two focal lengths.

Grating surface 1062 is preferably adapted to optimally diffract thewavelength of radiation in reflected radiation beam 23R and is ruled sothat the radiation incident on the grating surface is diffracted toreturn along the path of incidence. The optimum grating period P isgiven by P=nλ/2sin φ_(G) where λ is the wavelength of the radiation andφ_(G) is the angle of incidence onto the grating relative to the gratingsurface normal N_(G), and n is the refractive index of the mediumsurrounding the grating (n=1 for air). The purpose of the grating is tocompensate for the tilted focal plane at the substrate, which wouldotherwise result in the return image being defocused by an amountdepending on the distance in the plane of FIG. 19 between the point 321and the axis of the relay 1050. Note that in this geometry, where relay1050 operates at −1×, φ_(G)=φ_(14B)=φ_(23R)=φ_(23RD). In general, tanφ_(G)=M tan φ_(23R) where M is the magnification of relay 1050 from thesubstrate to the grating.

In operation, reflected radiation 23R is collected by telecentric relay1050, which includes lens 1070 and lens 1072, which brings the radiationto a focus onto grating surface 1062. Grating surface 1062 redirects (ormore precisely, diffracts) the radiation back to relay 1050, whichdirects what is now recycled radiation 23RD back to substrate surface 62at or near the point 321 where the reflected radiation originated.

A shortcoming with the embodiment of FIG. 19 is that reflected radiation23R is imaged onto a very small spot on the grating on a continuingbasis which can eventually melt or otherwise damage the grating. Asimilar problem would be encountered using a normal-incidence mirror(not shown) in place of the grating. Therefore, care must be taken inoperating apparatus 10 using the example embodiment, of recyclingoptical system 900 of FIG. 19.

FIG. 20 is a cross-sectional schematic view of an example embodiment ofa laser scanning apparatus for annealing substrate 60, wherein theapparatus employs two optical systems 20 and 20′ having associatedtwo-dimensional laser diode array radiation sources 12 and 12′,respectively, arranged along respective axes A1 and A1′, respectively.Continuous radiation sources 12 and 12′ are both operatively connectedto controller 70 and emit radiation beams 14A and 14A′, respectively.Each radiation beam is received by the corresponding optical system 20and 20′. Optical systems 20 and 20′ form from radiation beams 14A and14A′ corresponding radiation beams 14B and 14B′ that in turn formrespective images 100 and 100′ at substrate surface 62.

In one example embodiment, optical systems 20 and 20′ are adapted toform images 100 and 100′ that at least partially overlap with oneanother at the substrate. In another example embodiment, images 100 and100′ are line images. In another example embodiment, at least one ofannealing radiation beams 14B and 14B′ is incident substrate surface 62at an incident angle φ_(14B) or φ′_(14B) that is at or near theassociated Brewster's angle φ_(B) for silicon.

Such an arrangement reduces the demands on the continuous radiationsources 12 and 12′ to output high-power radiation beams 14B and 14B′.The example embodiment of the apparatus of FIG. 20 is not limited to tworadiation beams 14 and 14B′. In general, any reasonable number ofcontinuous radiation sources 12, 12′, 12″, etc., and correspondingoptical systems 20, 20′, 20″, etc. can be used to form correspondingimages 100, 100′, 100″, etc., (e.g., line images) on substrate surface62 to achieve the desired annealing effect.

The many features and advantages of the present invention are apparentfrom the detailed specification, and, thus, it is intended by theappended claims to cover all such features and advantages of thedescribed apparatus that follow the true spirit and scope of theinvention. Furthermore, since numerous modifications and changes willreadily occur to those of skill in the art, it is not desired to limitthe invention to the exact construction and operation described herein.Accordingly, other embodiments are within the scope of the appendedclaims.

1. An apparatus for thermally processing a region of a substrate,comprising: a continuous radiation source capable of providing acontinuous first radiation beam having a wavelength capable of heatingthe substrate region; an optical system adapted to receive the firstradiation beam and form a second radiation beam therefrom that forms animage at the substrate; a recycling optical system arranged to receiveradiation reflected from the substrate and direct the reflectedradiation back to the substrate as a recycled radiation beam thatapproaches the substrate region at a non-normal incident angle selectedaccording to the substrate region to form a reflected image having thesame orientation as the image; and a stage adapted to support thesubstrate and scan the substrate relative to the image to heat theregion with a first pulse of radiation from the optical system and asecond pulse of radiation from the recycling optical system to atemperature sufficient to thermally process the region.
 2. The system ofclaim 1, wherein the recycling optical system includes acollecting/focusing lens and a corner cube reflector.
 3. The system ofclaim 2, wherein the recycled radiation beam and the second radiationbeam have respective incident angles, the recycling optical system hasan optical axis, and wherein the corner cube reflector is displacedrelative to the optical axis so as to at least partially separate theincidence angles of the recycled and second radiation beams.
 4. Thesystem of claim 1, wherein the recycling optical system includes atelecentric relay and a diffraction grating.
 5. The system of claim 1,wherein the recycling optical system includes in order from thesubstrate along an optical axis: a cylindrical mirror; aunit-magnification (1×) relay; and a polarization-preserving roof mirroradapted to reflect the reflected radiation back through theunit-magnification relay back to the substrate.
 6. The system of claim5, wherein the unit-magnification relay includes: first and secondcylindrical lenses having the same focal length and separated by theirtwice the focal length; and a pupil half-way between the first andsecond cylindrical lenses.
 7. The system of claim 1, wherein therecycling optical system is adapted to direct the recycled radiationbeam to the substrate at an incident angle at or near the Brewster'sangle.
 8. An apparatus for thermally processing a region of a substrate,comprising: two or more continuous radiation sources each capable ofproviding a continuous first radiation beam having a wavelength capableof heating the substrate region; respective two or more optical systemseach adapted to receive a corresponding one of the first radiation beamsand form a second radiation beam therefrom that forms an image at thesubstrate, thereby forming respective two or more images at thesubstrate; and a stage adapted to support the substrate and scan thesubstrate relative to the two or more images to heat the region withrespective two or more pulses of radiation to a temperature sufficientto thermally process the region, wherein the apparatus is constructedsuch that each second radiation beam approaches the substrate region ata non-normal incident angle selected according to the substrate regionwith each of said respective two or more images having the sameorientation.
 9. The apparatus of claim 8, wherein the two or moreoptical systems are adapted for form said respective two or more imagesas line images.
 10. The apparatus of claim 1 wherein the recycledradiation beam is formed from parallel rays.